Transceivers which are located inside vehicles are commonly connected to an antenna which is mounted on the exterior of the vehicle by means of a coaxial cable or other wire link. Frequently, the radiating and receiving element of the antenna, which is located on the exterior of the vehicle, is capacitively coupled to the coaxial cable termination through a glass window of the vehicle, thereby eliminating the necessity of drilling holes in the body of the vehicle.
The increasingly common use of cellular telephones operating in the 800 to 1000 MHz frequency range in motor vehicles has promoted the use of such through the glass antenna units since the cellular telephone preferably utilizes an antenna whose mast extends above the roof line of the vehicle for optimum reception and transmission. Through the glass antennas are easily mounted near the top of the rear window and the antenna mast can extend vertically above the roof line.
Several types of cellular telephones are common today. A permanently installed car telephone has a direct power connection to the vehicle electrical supply and has a coaxial link to the installed antenna. A so called "transportable" cellular telephone is a similar telephone unit which includes a self contained power supply and a movable antenna so that it can be carried in a brief case. The permanent and transportable telephones are permitted to have a maximum transmitted power of 3.0 watts, which generally mandates the use of a coaxial transmission line to an antenna.
In recent years, a smaller, compact and lightweight cellular telephone has been developed which can be hand held. This hand held or "portable" telephone, which usually has an integral antenna as a part of the unit, is permitted a radiated power level of only 0.6 watts. Such devices can be quite small and can fit in one's pocket. When used in an open space, the portable can easily communicate with a "cell" of the cellular system. However, difficulties can be encountered if one wishes to use a portable when inside a vehicle since the metal body of the vehicle acts as a shield to both incoming and outgoing signals.
These difficulties can be overcome if the portable unit can be connected to an exterior antenna or if the portable can be operated through an open window in the vehicle and the metallic mass of the vehicle doesn't affect the receiving or radiation patterns of the antenna. One approach has been marketed under the trademark LARSEN.RTM. ANTENNAS by Larsen Electronics, Inc., of Vancouver, Wa., Model KGB-825. This unit is described as a "passive repeater antenna" which passes signals to and from the externally mounted gain antenna elements.
Such an approach, however, fails to consider the low power available from the portable phone unit and the fact that the radiation pattern from the portable phone antenna is omnidirectional. This generally results in a very small fraction of the radiated power reaching the "repeater" and the external antenna unit. Similarly, the energy received by the external antenna is transferred to the "repeater" and omnidirectionally radiated within the vehicle. Only a small fraction of the received is signal is acquired by the portable phone antenna. Further, the external antenna and the internal dipole repeater are coupled capacitively, through the glass window, thereby resulting in some signal loss.